Cleaning up the effects of stormwater with biofilters

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Source: European Commission, Environment DG

Stormwater in urban areas can have negative effects on both the quantity and quality of surface- and ground-waters. Biofiltration systems are a popular form of water treatment, but information on their performance with stormwater is limited to laboratory studies. This research is the first to evaluate three biofiltration systems in practice. Biofiltration systems (otherwise known as biofilters, bio retention systems and rain gardens) clean water by filtering diverted runoff through dense vegetation and layers of natural substances, such as sandy soil, compost or shredded hardwood.

Biofilters are popular for their ability to reduce flood peaks and remove pollutants. They also have a small spatial footprint and can be landscaped into the surrounding area. The EU Floods Directive1 aims to reduce the risks posed by floods and one of the main goals of the Water Framework Directive2 is to clean up Europe's waters. Successful biofiltration systems could help member states with both these initiatives.

The research evaluates the effectiveness of three field-scale biofiltration systems. Flood conditions were simulated and the researchers recorded the ability of the systems to reduce run-off flow rates, to retain water and to remove pollutants. Pollutants included heavy metals such as zinc, copper and lead and nutrients such as phosphorus and nitrogen. Nutrients are detrimental to water quality in large quantities.

Biofilters were shown to effectively reduce peak runoff flow rates by at least 80 per cent. They were also shown to reduce runoff volumes by 33 per cent on average. It is important to note that this result was for a lined system, where water cannot filter into to surrounding soils - reductions in runoff volumes are likely to be higher for unlined systems. The study also demonstrated that vegetation is important for maintaining the amount of water that could pass through the system (hydraulic capacity), because roots help prevent the compacting and clogging of the filter material whether it is soil, compost, sand etc. However, the data indicated that the better the filter at maintaining water flow, the higher the concentration of some pollutants. Therefore there may be a trade-off between hydraulic capacity and pollutant removal.

Suspended solids and heavy metals were effectively removed in all filter designs, with reductions generally in excess of 90 per cent. In contrast, removal of nutrients such as phosphorus and nitrogen was variable, and ranged from consistent leaching to reliable removal, depending on the design. On the basis of their results the researchers suggested that a filter with a low phosphorus content itself would be more effective at removing phosphorus pollution. The removal of nitrogen, however, is more difficult because it is highly soluble and influenced by the wetting and drying regime of the biofilter operation.

By studying the biofilters at work, the study demonstrates their effectiveness but also indicates that the choice of design depends on the site and the target pollutants. It should be noted that biofilters are generally only designed to manage small to medium sized floods and the research was designed to simulate this type of rainfall event.

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